The goals of this project are (1) to understand better the dynamic interactions between the voltage-gated Na channel and specifically bound ligands and (2) to characterize the unique conformational transitions of the ligand-modulate voltage-gated Na channel. We plan to examine the pharmacological properties of local anesthetics, Alpha-, and Beta-scorpion toxins on the voltage-gated Na channel at the level of macroscopic currents in axons and, subsequently, at the level of single channel events in cultured cells. The kinetics of these current records reflect the conformational transitions among resting, open, inactivated, and several intermediate states of the Na channel. The existence of many conformational states requires complicated structural rearrangements of the Na channel polypeptide following membrane depolarization. Perturbation of these structural rearrangements upon binding with a ligand may stabilize certain conformations and thus provide enlarged opportunity to study the properties of these conformations. Reciprocally, the ligand binding site may change its configuration during the structural rearrangement of the channel and thus alter its binding affinity with the ligand. This general description of channel-ligand interactions is now known as Hille's modulated receptor hypothesis. The usefulness of this hypothesis will be tested by using three different classes of ligands as molecular probes: local anesthetics which block the Na currents, Alpha-scorpion toxins which specifically modify Na channel inactivation, and Beta-scorpion toxins which primarily modify Na channel activation. Both the actions of the native ligands and of their various derivatives, including the photoactivatable derivatives which can be covalently bound to the channel by phase-locked photoactivation procedures, will be studied during voltage-clamp steps of varying magnitude and duration. The resulting dynamics of ligand-receptor interactions and their underlying molecular mechanisms should provide novel information about the microscopic processes occurring during channel gating. Furthermore, affinity coupling of these ligands or their derivatives on the primary sequence of the channel polypeptide may in the future permit the direct identification of a known region of the channel with a special function.